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Generating AC power onboard
Many of the electrically powered devices on our boats
can operate directly from 12 volt, DC battery power. Electronic
instruments, autopilots, cabin lights, engine start, and many other
devices use 12 volts. However these days our boats are filled with all sorts of
modern equipment, from air conditioning, microwaves, TV, refrigeration,
computers etc all of which require 120-volt, 60Hz shore power (US).
In this article we will discuss the various methods of generating AC power on a boat. Each has its own features and associated costs.
First we look at how much energy you will need How to choose generating equipment largely depends on the wattage you will need to power your onboard equipment. Here are some devices and approximate power consumption. These are generic numbers so check the models you have. Heat generating devices consume the most power. Maybe means that if you have a large enough inverter and battery bank and do not run the device for long, an inverter may work. You will need to calculate starting up watts in the total watt calculations. A 100 watt TV may use 500 watts at startup. If you do not have those 500 watts available, the TV will not start. Air conditioners have large start up power requirement, which means you need to look at that not just the continuous running load. 
Inverters transform a boats 12-volt DC battery power into AC. Inverter models range from 3000 watts to less than 100 watts, and depend on the amount of battery power available. Portable inverters go up to around the 300-500 watt range. For larger inverters you will need to mount them in a cool, dry, well-ventilated space close to the battery bank. Inverters come in modified and pure sine water options. Pure sine wave is recommended for running entertainment systems and computers etc. Modified sine wave may be good for some TV sets and many radios. Your household AC is pure sine wave. To pick an inverter there are two questions. First, what AC appliances are you going to run? Add up the wattage for each appliance and chose an inverter that delivers more watts of continuous power. It is no good having a 1000 watt inverter if the appliances you run eat up more than 1000w (assuming they are running at the same time). A 7 amp microwave @ 115V will need 805 watts, using Volts x Amps = Watts. To size an inverter add up the load requirements, see table A for ideas.
Xantrex PROwatt Sine Wave Inverter. The new PROWatt SW inverter from Xantrex is a compact true sine wave inverter. PROwatt inverters come in 800, 1000 and 2000 watts. This inverter is perfect for powering Cameras, cell phones, computers and other portable equipment. The inverter measures 3.5 x 8.7 x 13.4" for the 1000 Watt model, which makes it small enough to fit in many places without taking up valuable space. This inverter is easy to use and easy to install, just grab some Heavy Duty Inverter Cables and connect to your battery.
Portable inverter example
This little inverter fits in a cup holder and plugs into a 12 volt receptacle (cigarette style plug. Powering up to 150 watts, the power inverter provides power for laptops, cell phone chargers, radios, portable TVs, camcorders, small power tools etc. At only $40 this is a good option.
Battery Management
Warning; with an inverter you can power your AC
appliances, quietly without fuss, however its will be too easy to drain your
battery bank. Therefore have some check mechanism, so your not left
helpless. A separate engine battery is a must.
Unfortunately inverters are power hungry and take more energy from the source (Batteries) than they supply in AC power. So how much are you going to drain the batteries? Inverters have alarms and automatic shutdown, so they will protect the inverter but not the battery. The ProSport 150 watt inverter above has a shut down of 10V DC and an alarm at 10.5V DC. This is fine for the inverter, but the batteries need a cut off voltage of 12.1-12.2 volts. Powering a 100 watt device from an inverter for 1 hour will drain around 10 amp hours from the Battery. Next you will need to recharge the battery and assuming a 15% energy loss you will need to charge 11.5 amp hours back into the battery. Battery management dictates we only discharge deep batteries to 50% max before recharging. Two group 27 lead acid flooded cell batteries have a combined 200 amp hours capacity. At 50% discharge, you have 100 amp hours to burn. If you run the engine the alternator will add back some amps to limit the battery discharge. It is also possible to add solar or wind power to replace battery amps.
 The simplicity of a portable generator like this is the ability to
directly plug in an appliance from lights, power tools, computers to
battery charging.Honda EU1000iA generator generator little Honda generators and the line are very quiet, self-adjusting to match the load, and easy to handle (the 1000 watt unit is 29 pounds, 43 for the 2000 watt). They also have a direct battery charge output. There is a special cable for charging batteries. If you were to keep this on the dock, you should run a 20amp or maybe larger cable with a power strip and built in circuit breaker on to the boat and use this to plug into. Do not get any water on any cables and do not use a household extension cord.Safety is a big concern with a portable generator. You have to think about CO, hot exhaust, and Fuel spills. Fuel storage, and maintenance are some of the other issues. To size a portable generator add up the load requirements, see table A for ideas.
Some equipment can be run from an engine driven alternator. An example of this is the Engine driven refrigeration. This can cool down your ice box while motoring and then keeps the ice box cool for long periods when the engine is not being used. Other devices like water heaters can do their job while you are using the engine, and hot water is available later at anchor. The idea being if you can limit the at anchor needs to small wattage devices you may get away with an inverter. This alternative works better for power boats, while sailboats may not have enough engine time. This will not help you with air conditioning however.
Warning; running the engine in neutral to power the alternator is not good for the life of the engine, and running the engine for an extended period of time on a mooring will not endear you to neighbors.
Shore Power may be the simplest way to get AC power onboard. You basically can run any device that you would have at home. Good reasons for having shore power include; you have a Marina which has Shore-Power pedestals, you use AC appliances at the dock. Even if you do not do any of the latter, you can still tie up, turn on the shore power and go home, leaving the shore power to keep your batteries fully charged. Check your Marina to see what amp service is available. A typical shore power systems consists of;
 Xantrex MS Series True Sine Wave Inverter/Charger Fixed-mount inverters are available with built-in shore-power switching systems. When an auto shore-power switching inverter is installed, shore-power flows through the inverter and is then delivered to the boats AC system. The inverter operates in standby mode when shore power is present. To size an inverter/charger add up the load requirements, see table A for ideas.
The simplest shore power system is an extension cord  Dockside 30A to 15A Adapter with GFCI If you don't have a built in shore power system, you can use this connector to the docks AC pedestal and run an extension chord to the boat.
If you live in an area which has very hot & humid temperatures then having a generator will let you run air conditioning at anchor. Also you will be able to run refrigeration and other devices too large for an inverter. Generators for recreational boats run from 3-15 KW and even higher if necessary. The good news is that as technology improves we are getting more KW, in a smaller package, quieter than we did 10 years ago. One of the biggest obstacles to having an onboard generator has been the size.
A genset will deliver as much AC power as you may need, for as long as you have fuel. Generators with sound shields can be very quiet, especially when fitted with exhaust systems with split cooling water & exhaust gas. As with all the AC power options you will need to add up your equipment loads to find out how large a genset you need. see table A for ideas.The biggest issue with a generator is the space required and the installation. The diagram below shows the installation for the red Westerbeke above. You will need thru hull fittings for water intake, and Exhaust system, muffler, anti siphon and then you need the wiring.
A typical genset suitable for an air conditioned 30-foot boat will cost about $4,500, and that does not include installation costs.
Used
Boat Parts
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Fixed inverter example
Typical specs of a shore power
installation; 30 amp 110VAC shore power system with 6 outlets, 50ft
shore power cord and ProMariner Galvanic Isolator with
